Co-extrusion of acrylic ester polymer/polystyrene multiple-layered sheeting

ABSTRACT

A process is disclosed for producing composite sheeting of polystyrene having a thin protective layer of an acrylic ester polymer. The process comprises joining a molten stream of each polymeric material in a single extruder discharge conduit such that there is produced a single molten stream having a reasonably well defined interface between the two types of polymeric material, and thereafter, passing the resulting single stream of material through a sheet-form extrusion die having its die lips generally aligned with the foregoing interface.

United States Patent [1 1 Wiley 1 May 6, 1975 l CO-EXTRUSION OF ACRYLIC ESTER POLYMER/POLYSTYRENE MULTlPLE-LAYERED SHEETING [75] Inventor: Donald F. Wiley, Big Spring, Tex.

[73] Assignee: Cosden Oil & Chemical Company, Big Spring, Tex.

221 Filed: Aug. 10, 1973 21 Appl. No.: 387,506

Related US. Application Data [63] Continuation of Ser. No. 128,724, March 29, i971,

abandoned.

[52] US. Cl. 264/171; 156/244; 264/173; 425/131 [51] Int. Cl. 829d 7/04 [581 Field of Search 264/l7l, 173; 425/l30, 425/l3l; 156/244 [56] References Cited UNITED STATES PATENTS 3,476,627 ll/l969 Squires 264/171 LeFevre et al 264/l7l Schrenk et a1. 264/l 7l ll/l969 4/1971 Primary Examiner.leffery R. Thurlow {S 7] ABSTRACT A process is disclosed for producing composite sheeting of polystyrene having a thin protective layer of an acrylic ester polymer. The process comprises joining a molten stream of each polymeric material in a single extruder discharge conduit such that there is produced a single molten stream having a reasonably well defined interface between the two types of polymeric material. and thereafter, passing the resulting single stream of material through a sheet-form extrusion die having its die lips generally aligned with the foregoing interface.

6 Claims, 3 Drawing Figures PATENTEUHAY BIHYS INVENTOII DONALD E WILEY M. NORWOOD CHEAIRS ATTOII N EYS 1 CO-EXTRUSION OF ACRYLIC ESTER POLYMER/POLYSTYRENE MULTIPLE-LAYERED SHEETING CROSS REFERENCE TO RELATED APPLICATIONS This is a continuation of application Ser. No. l28,724, filed Mar. 29, 197], now abandoned.

Our copending applications Ser. No. l28,939 filed concurrently herewith, and Ser. No. 368,834 filed June 11, 1973, both assigned to the assignee of this application.

BACKGROUND OF THE INVENTION This invention relates to the production of multiplelayer sheeting, and more especially, to the extrusion of polystyrene sheeting having a thin protective layer of an acrylic ester polymer.

Co-extrusion processes for the production of multilayer sheeting are of course well known in the art however, hitherto known processes almost exclusively employ some form of encapsulation technique wherein one stream of thermoplastic material, typically the volumetrically smaller stream, is completely surrounded, e.g., coaxially, by a second stream of such material prior to passing the entire composite stream through an extrusion die. Alternatively, the foregoing encapsulation may be effected in the cavity-portion of an extrusion die. In both types of process, the resultant sheeting product is characterized by an inner-layer of one type of thermoplastic material sandwiched between or encapsulated by two exterior layers of a second thermoplastic material.

Very little success, with one possible exception, has been recorded in the area of co-extrusion of multiple thermoplastic resinous layers merely laminated on top of one another. Most attempts have employed a series of extrusion dies whereby two distinct sheets of different material are first formed by extrusion, superimposed upon one another and then passed through yet another extrusion die or other restrictive orifice. In other attempts, there are employed complex extrusion dies wherein individual molten streams of thermoplastic material are superimposed upon one another in the die cavity immediately before passing through the die lips.

The possible exception referred to hereinabove relates to US. Pat. No. 3,476,627 which discloses a process comprising combining two streams of molten thermoplastic resin in a conduit upstream of an extrusion die such that they have a sharply defined juncture plane, and thereafter passing the composite stream through an extrusion die in such a manner that the juncture plane is parallel to the principle direction of flow of the resin as it takes on the shape of sheeting. The foregoing disclosure is concerned primarily with the co-extrusion of multiple-layered polyvinyl butyral resin. This patent does, however, refer broadly to a process for co-extruding multiple-layered sheeting of different thermoplastic resins having similar processing characteristics and which will adhere to one another, and further limited by the requirement that the volume ofa given resin component be at least 25% of the total resin content.

In contrast, this invention is concerned with the pro duction of multiple layered sheeting of two materials which have different processing characteristics, which are highly incompatible and which have hitherto defied attempts both through co-extrusion techniques and conventional laminating methods to produce a strongly adhering bond therebetween. Prior attempts aimed at co-extrusion of such materials have resulted in multilayered products wherein the individual layers are rather easily peeled from one another, and like attempts to laminate materials of this nature by extruding a molten layer of one polymer onto a preformed sheet of a second polymer have provided similar unsatisfactory results. Moreover, the invention relates to multilayered sheeting consisting primarily of a less expensive material, i.e., polystyrene, and having only a very thin surface layer or veneer of a second, more expensive polymeric material having highly desirable surface characteristics, i.e., an acrylate ester polymer.

SUMMARY OF THE INVENTION It is therefore a primary object of the invention to provide a process for producing in a single extrusion step multiplelayered sheeting from two or more normally incompatible polymeric materials.

A further object of the invention resides in providing a method for producing multilayer sheeting having a major thickness of a less expensive polymeric material and a relatively thin surface layer of a more expensive polymeric material which exhibits highly desirable surface characteristics.

A specific object of the invention lies in the production of a multiple-layered sheet comprised primarily of polystyrene and having a protective surface layer of an acrylic ester polymer.

Other objects, features and advantages of this invention will become apparent from the description which follows, when considered in view of the accompanying drawings.

Thus, in accomplishing the foregoing objectives, there is provided according to the invention a process for producing composite sheeting of polystyrene having a relatively thin protective layer of an acrylic ester (acrylic) polymer. The process comprises joining a molten stream of polystyrene and a molten stream of acrylic in an extruder discharge conduit to form a single stratified stream of molten material conforming to the crosssection of the conduit and having a relatively sharply defined interface between the polystyrene and acrylic, and thereafter passing this stratified stream through a sheet-form extrusion die having its die lips generally aligned with the foregoing interface. Firm adherence between the polystyrene and acrylic layers is achieved by providing that the melt viscosity of the acrylic be relatively close to that of the polystyrene, and advantageously nearly identical thereto at the junction point of the two polymeric streams. In this process, the relative proportion of acrylic polymer is preferably less than about 20% by volume based upon the total polymeric material. Important operating parameters include the temperature at which each polymeric material exits from the extruder and also the temperature of the extrusion die.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 schematically represents an apparatus for the preparation of a multiple-layered sheet or film employing the method of the invention.

3 FIG. 2 is a cross-sectional view taken along the line 2 2 in H6. 1.

HO. 3 is a top view, in section, of the extrusion die and lead-in conduit portion of the apparatus illustrated 4 inherent melt viscosity relatively approximate to that of the polystyrene chosen for the substrate, or alternatively an acrylic ester polymer having a melt viscosity which may be adjusted under extrusion conditions, as

in FIG. I. 5 for example by sufficiently raising the temperature of FIG. 4 is a cross-sectional view taken along the line the polymer, to a value approximating the melt viscos- 4 4 in FIG. 2. ity of the polystyrene. While it is not possible to put precise limits on the degree of similarity required be- DETAILED DESCRIPTlON OF THE INVENTION tween the respective melt viscosities of the polymeric in accordance with this invention it has been discovl0 materials, and although there perhaps will be some deered that a multiple-layered sheet or film may be progree of unpredictability between particular polymers duced from two polymeric materials which have hithfalling within any broad characterization, in general, erto been found extremely difficult to unite by convensatisfactory results are achieved in accordance with the tional extrusion techniques or by postextrusion lamiinvention if the melt viscosity of the acrylic is within the nating procedures. Specifically, the sheeting or film of range of about 50 to l50% of that exhibited by the polthe invention consists of a layer of polystyrene having ystyrene. These relative values are only important a thickness substantially corresponding to the final under actual co-extrusion conditions since in most incomposite sheeting thickness desired, and adhered to tan a r li ester polymers which differ from polyeither one Or both ides Of t p ly yr n l y r. a relastyrene in melt viscosity under standard test conditions tively thin exterior layer of an acrylic ester polymer. b m re than the above range can be modified under Thus, there is P o ided composite sheeting having the processing conditions to adjust the melt viscosity into desirable economic characteristics of polystyrene and suitable proximity to that of the polystyrene sublayer concurrently possessing the highly desirable surface material. To illustrate the relationship between relative properties of acrylic polymers. Such stock material is melt viscosity and adherence of acrylic and polystyrene characterized by good weather resistance and a high layers, the following comparative results are presented degree of oxygen and water vapor impermeability. for representative co-extruded sheets:

Melt Viscosity Values Melt Flow Rate By Extrusion Plasto- Brabendcr meter (A.S.T.M. D- Torque l238-63T) Condition [0 min.) Polymers G (Gms/IO minutes) Meter-Grams Adherence l. Acrylic Ester- Rohm 8!. Haas Type VM Plexiglas 2.35 580 Good Polystyrene- Cosden Oil and Chemical Co. Impact 825D pellets 2.0 3.0 370 Rohm & Haas Type V-8l l Plexiglas (U.V. Absorber) .90 860 Fair-Poor Em Cosden Oil and Chemical Co. 2.0 3.0 370 lmpact 825D pellets Thus, the materials are particularly useful for fabrication of food packaging containers and building products.

An important aspect of the invention resides in the discovery that an essentially uniform and relatively thin layer of an acrylic may be firmly united to the polystyrene sublayer in a single extrusion step. Although similar extrusion processes are known in the art, as evidenced by the above-mentioned U.S. Patent, the same relate to extrusion of chemically similar or at least compatible resinous materials, whereas the process of this invention has been successfully applied to chemically and physically dissimilar resins. Specifically, it has been discovered that polystyrene and an acrylic can be coextruded to produce a multilayered sheet exhibiting strong adhesion between acrylic and polystyrene layers if there is chosen an acrylic ester polymer which has an in another important aspect, the present extrusion process has been found operable to produce multilayered sheeting having extremely thin surface layers of acrylic polymer, e.g., on the order of 1 mil or less, in comparison to the total sheet thickness, e.g., 10 to 12 mils or thicker. This is achieved by providing relative feed rates of the polystyrene and acrylic such that the volume of acrylic polymer generally does not exceed about 25% and preferably is less than about 20% of the total resin feed. More preferably, the acrylic component constitutes less than about l5% of the total extrudate. This is in direct contradistinction to the prior art which discloses operability only above a minimum resin content for any given resin component of at least 25% by volume.

The principles of the invention are generally applicable to the manufacture of either multilayer polymeric film (less than mils in thickness) or sheeting (10 mils or thicker); however, polystyrene based film has not found widespread popularity because of the difficulties encountered in adapting this materialto blown film apparatus conventionally employed together with extrusion equipment in the manufacture of film products. Thus, broadly speaking, it is possible to produce a composite film having a l mil or thinner surface layer of an acrylic polymer on a 4 or 5 mil, or even a 1 mil base layer of polystyrene. But practically speaking, the products of most interest are multilayer sheets of from 10 mils up to inch in thickness having acrylic surface layers measuring from a fraction, e.g., or A mil, to several mils. Accordingly, it will be appreciated that volumetric feed rates for the acrylic polymer component are frequently very small, e.g. less than 1%, when thicker sheeting is desired, and likewise that rates in excess of 20 or 25% are also contemplated when very thin film is produced.

Referring now to the drawings, in FIG. 1 there is schematically illustrated an apparatus, generally designated by the reference numeral 10, particularly adapted for the process of the invention. The apparatus 10 comprises in cooperative combination a first extruder 11 for the extrusion of molten polystyrene, and connected thereto, a discharge conduit 12. A second extruder 14 having a discharge conduit 15 is adapted to provide a minor stream of molten acrylic polymer. Conduit 15 terminates at conduit 12 at a point upstream of the sheeting die 17 which is in operative communication with conduit 12 and receives the flow therefrom. Sheet 19 is formed at the die lips 18 and thereafter carried from the die by means of polished cooling rollers 21, 22 and 23.

In FIG. 2 there is illustrated a sectional view of the discharge conduits 12 and 1S, extrusion die 17 and cooling roller arrangement 21, 22 and 23. This view illustrates the intersection of conduits 12 and 15, and the configuration of the polystyrene resin 31 and acrylic resin 32 as they are combined in conduit 12 to form a stratified stream having an interfacial juncture plane 25. Stratification at the intersection point of the two resins is aided by metal plate 34 positioned in conduit material maintain their stratified relationship, despite the minute proportion of acrylic polymer, to form a final product having an essentially uniform surface layer of this material.

In FIG. 3 is illustrated a top view of the conduit 12 and extrusion die 17 to demonstrate the lateral flow pattern of molten polymeric material as it passes into and through the extrusion die. FIG. 4 illustrates more clearly the preferred configuration of metal plate 34 positioned on conduit 12.

To produce a multi-Iayered sheet or film having a satisfactory degree of adherence between layers, it is necessary to observe certain process limitations during the extrusion procedure. While it is of course desirable to provide relatively steady fiow of polystyrene and acrylic through the apparatus and to avoid any turbulence therein, these limitations are rather typical in most extrusion processes employing highly viscous synthetic resins. More important limitations (although not wholly unrelated to the foregoing) reside in the extrusion temperatures for the polystyrene and acrylic polymer, and the temperature maintained in the extrusion die. The extrusion of the polystyrene from extruder 11 into conduit 12 should be carried out at a temperature of from about 400F. to about 500F., whereas the acrylic should be discharged from extruder 14 at a temperature of from about 450F. to about 570F. The role of the latter range will be appreciated in view of the foregoing comments relative to the desired melt viscosity of the acrylic at the time of co-extrusion. Hence. this range is not intended to be an absolute limitation on the invention, but rather indicates only the median range wherein the melt viscosity of acrylic resins may be rendered proximate to the melt viscosity of polystyrene so as to provide strong adherence between layers in the extruded sheet. It is to be understood that the invention would likewise embody within its scope the use of acrylic resins which might be extruded at even higher temperatures to produce satisfactory adherence to the polystyrene substrate co-extrudate.

As indicated, die temperature is also an important process variable, but not from the standpoint of layer adherence to one another. Instead, this parameter affects the surface characteristics, eg. gloss, etc., of the extruded sheet. Die temperature should be maintained relatively constant during extrusion, with typical values thereof for co-extrusion of acrylic and polystyrene ranging between about 420 and 500F. The best surface gloss is attained with temperatures above about 450F.

In the co-extrusion of polystyrene and acrylic ester polymers, the conditions maintained at the cooling roller assembly are of little importance as regards the surface properties of the final product. The manufacture of smooth sheeting normally requires the use of highly polished rollers, e.g., chrome, typically three in number, each being approximately 12 inches in diameter and being adapted for internal circulation of cooling water. When an acrylic is co-extruded as a surface layer, it has been found that roll temperatures of about F., and roll pressures approximately enough to overcome upward spring tension on the rollers produce acceptable surface characteristics, especially gloss.

Other processing variables of lesser importance include the pressures at which, the polystyrene and acrylic are separately extruded before combination. These pressures are of little significance and typically fall within the range of 750 and 3000 psi. for the polystyrene stream and similarly between about 500 and 3000 psi. for the acrylic polymer stream. Of course, the downstream pressure at the point where the streams intersect is equal in both streams.

The term polystyrene as employed herein includes both homopolymers of styrene and copolymers of styrene with other polymerizable and polymerized mono mers. Included within the latter category are impact polystyrenes which comprise graft copolymers of styrene upon conjugated diene backbone polymers such as polybutadiene, butadiene-styrene copolymers, butadiene acrylonitrile copolymers, natural rubber, etc. Likewise included in this category are normal copolymers of styrene with other well known, conventional monomers.

Similarly, the term acrylic ester polymer is intended to connote a broad class of polymeric materials based upon acrylic and/or methacrylic acid esters. The lower alkyl esters constitute the preferred class of materials. Included within the intended definition are homopolymers of acrylic and methacrylic acid esters as well as interpolymers thereof with minor amounts of compatible copolymerizable monomers, most commonly other acrylic monomers, e.g., acrylic acid, methacrylic acid, acrylonitrile, acrylamide, etc. as well as monomers of the styrene family. Also encompassed within the definition are blends and/or grafts of acrylic ester polymers with additional modifying polymers, such as rubbery impact modifiers. An important class of such blended polymers comprises acrylic ester homoor inter polymers blended with graft copolymers of acrylic ester, and optionally other monomers, upon a rubbery backbone polymer. See for example US. Pat Nos. 3,261,887, 3,354,238 and 3,475,516.

A multitude of acrylate ester polymers are available commercially. An example of the poly-blended acrylic polymers mentioned above are the acrylic polymers marketed by American Cyanamide under the trade designation XT Multipolymers. Suitable acrylic resins for use in the process of the invention may easily be ascertained by one of ordinary skill in the art from molecular weight, melt index and/or melt viscosity data characterizing any given resin.

It will be appreciated that multilayer sheeting having a surface layer of acrylic on both sides of the polystyrene layer may be formed according to the principles of the invention. Such an embodiment merely requires that various minor modifications be made in the physical equipment employed to carry out the process. Likewise, it will be appreciated that various types of extrusion dies may be substituted for the one presently illustrated, e.g., end-fed die.

The following specific example is provided to facilitate a better understanding of the invention, it being understood that the same is intended to be merely illustrative and in no wise limitative.

EXAMPLE A main resin stream of impact polystyrene (Cosden Oil 8: Chemical Co. 825D pellets) is extruded from a 4% inch diameter two stage vented extruder containing a 4:1 compression ratio screw. A 1% inch diameter 24: single stage vented side extruder also having a 4:1 compression ratio screw is arranged as illustrated in FIGS. 1 and 2 and supplies a second stream of acrylate ester polymer (Rohm & Haas Co. Type VM Plexiglas (methyl methacrylate based polymer) Melt Flow Rate by Extrusion Plastometer (A.S.T.M. D-l238-63T) Condition G=2.35 grams/l min.; Brabender Torque=580 meter grams (10 min.)). The polystyrene is extruded at a temperature of 470F. and a feed rate of approximately 680 lbs/hr. The acrylic ester side stream is discharged from the /4 inch extruder at 500F. and at a feed rate of approximately 30 lbs/hr.

The two resinous streams are then combined in the discharge conduit of the 4% diameter extruder utilizing a baffle plate arrangement illustrated in FIG. 2. There results a single Stratified bi-component stream having a horizontal juncture plane between the polystyrene and acrylic. The bi-component stream is conducted to a center-fed sheet-form extrusion die which opens to an extrusion slit 37 inches wide with its lips set at approximately 59 mils. The die temperature averages about 420F.

Upon leaving the die lips, the extruded sheet passes through a series of three 12 inch polished chrome cooling rolls, the top roll maintained at l25F., middle roll at 150F. and bottom roll at F. Roll pressures at top and bottom are maintained at approximately 22 psi. at a gap setting of 53 mils. The polishing and subsequent rubber rolls are operated at about 8% excess speed to stretch the extruded sheet to a final thickness of 55 mils.

Examination of the final sheeting product evidences an essentially uniform layer of acrylic polymer approximately 2 /2 mils thick firmly adhered to the impact polystyrene base layer. This thickness corresponds closely with the relative feed rates for the two resins, i.e., approximately 4.4% acrylic.

Thus, there has been provided according to the invention a process for co-extruding multi-layered sheeting of polystyrene and an acrylic ester polymer wherein an acrylic layer is firmly adhered to at least one side of the polystyrene base layer. Moreover, the subject process enables the production of multilayered film or sheeting of polystyrene having very thin, e.g., less than 1 mil, surface layers of an acrylic polymer.

While the fundamental novel features and advantages of the invention have been pointed out in connection with a single illustrated embodiment thereof, it will be appreciated that various obvious modifications of the co-extrusion process will suggest themselves to one of ordinary skill in the art. Therefore, it is intended to be limited only by the scope of the following claims.

What is claimed is:

l. A process for the manufacture of composite sheeting having a major layer corresponding substantially to the final composite sheeting thickness of a styrene polymer selected from the group consisting of homopolystyrene, impact polystyrene and normal copolymers of styrene containing a minor amount of another copolymerizable monomer, and a minor layer constituting less than 20% of the total final thickness of an acrylic ester polymer firmly united to the exterior surface of said major layer, comprising conveying a heat plasticized stream of said styrene polymer in a conduit having a generally circular cross section, joining to the exterior surface of said styrene polymer stream within the said conduit 21 second, heat plasticized stream of said acrylic ester polymer, said second stream constituting less than about 20% by volume of the combined first and second streams thereby forming a single strati fied stream of heat plasticized material conforming to the cross section of said conduit and being characterized by distinct, contiguous layers of said polymeric materials having a relatively sharply defined interface therebetween, said interface terminating at each end at a point on the inside surface of said conduit. adjusting the respective temperatures of the first and second streams under co-extrusion conditions to temperatures at which the melt viscosity of the acrylic ester polymer stream exhibits a value within the range of about 50 to 150% of the melt viscosity exhibited by said styrene polymer stream under the conditions of A.S.T.M. Test D-l238-63T, conveying said composite stream undisturbed through said conduit to a sheet-form extrusion die having its die lips generally aligned in parallel with the said interface between the two polymeric materials, and passing said composite stream of molten material through the said die to form a sheet, whereby said second stream spreads laterally across said first stream thereby forming said minor layer of said composite sheeting.

2. The process as defined by claim 1, wherein said styrene polymer is homopolystyrene.

3. The process as defined by claim 1, wherein the relative proportion of acrylic ester polymer is less than about 15% based upon the total polymeric material.

4. The process as defined by claim 1, wherein said interface has a generally arcuate shape with its arc of curvature being concave in the direction of the acrylic ester polymer stream.

5. The process as defined by claim 1, wherein the molten stream of acrylic ester polymer is extruded at a temperature of from about 450F. to about 570F. and the molten stream of polystyrene is extruded at a temperature of from about 400F. to about 500F.

6. The process as defined by claim 5, wherein the extrusion die is maintained at a temperature of from about 420F. to about 500F. 

1. A PROCESS FOR THE MANUFACTURE OF COMPOSITE SHEETING HAVING A MAJOR LAYER CORRESPONDING SUBSTANTIALLY TO THE FINAL COMPOSITE SHEETING THICKNESS OF A STYRENE POLYMER SELECTED FROM THE GROUP CONSISYING OF HOMPOLYSTYRENE, IMPACT POLYSTYRENE AND NORMAL COPOLYMERS OF STYRENE CONTAINING A MINOR AMOUNT OF ANOTHER COPOLYMERIZABLE MONOMER, AND A MINOR LAYER CONSTITUTING LESS THAN 20% OF THE TOTAL FINAL TICKNESS OF AN ACRYLIC ESTER POLYMER FIRMLY UNITED TO THE EXTERIOR SURFACE OF SAID MAJOR LAYER, COMPRISING CONVEYING A HEAT PLASTICIZED STREAM OF SAID STYRENE POLYMER IN A CONDUIT HAVING A GENERALLY CIRCULAR CROSS SECTION, JOINING TO THE EXTERIOR SURFACE OF SAID STYRENE POLYMER STREAM WITHIN THE SAID CONDUIT A SECOND HEAT PLASTICIZED STREAM OF SAID ACRYLIC ESTER POLYMER, SAID SECOND STREAM CONSTITUTING LESS THAN ABOUT 20% BY VOLUME OF THE COMBINED FIRST AND SECOND STREAMS THEREBY FORMING A SINGLE STRATFIED STREAM OF HEAT PLASTICIZED MATERIAL CONFORMING TO THE CROSS SECTION OF SAID CONDUIT AND BEING CHARACTERIZED BY DISTINCT, CONTIGUOUS LAYERS OF SAID POLYMERIC MATEIRALS HAVING A RELATIVELY SHARPLY DEFINED INTERFACE THEREBETWEEN SAID INERTFACE TERMINAATING AT EACH END AT A POINT ON THE INSIDE SURFACE OF SAID CONDUIT ADJUSTING THE RESPECTIVE TEMPERATURES OF THE FIRST AND SECOND STREAMS UNDER CO-EXTRUSION CONDITIONS TO TEMPERATURES AT WHICH THE MELT VISCOSITY OF THE ACRYLIC ESTER POLYMER STREAM EXHIBITS A VALUE WITHIN THE RANGE OF ABOUR 50 TO 150% OF THE MELT VISCOSITY ESHIBITED BY SAID STYRENE POLYMER STREAM UNDER THE CONDTIONS OF A.S.T.M. TEST D-1238-63T, CONVEYING SAID COMPOSITE STREAM UNDISTURBED THROUGH SAID CONDUIT TO A SHEET-FORM EXTRUSION DIE HAVING ITS DIE LIPS GENERALLY ALIGNED IN PARALLEL WITH THE SAID INTERFACE BETWEEN THE TWO POLYMERIC MATERALS, AND PASSING SAID COMPOSITE, STREAM OF MOLTEN ATERIAL THROUGH THE SAID DIE TO FORM A SHEET, WHEREBY SAID SECOND STREAM SPREADS LATERALLY ACROSS SAID FIRST STREAM THEREBY FORMING SAID MINOR LAYER OF SAID COMPOSITE SHEETING.
 2. The process as defined by claim 1, wherein said styrene polymer is homopolystyrene.
 3. The process as defined by claim 1, wherein the relative proportion of acrylic ester polymer is less than about 15% based upon the total polymeric material.
 4. The process as defined by claim 1, wherein said interface has a generally arcuate shape with its arc of curvature being coNcave in the direction of the acrylic ester polymer stream.
 5. The process as defined by claim 1, wherein the molten stream of acrylic ester polymer is extruded at a temperature of from about 450*F. to about 570*F. and the molten stream of polystyrene is extruded at a temperature of from about 400*F. to about 500*F.
 6. The process as defined by claim 5, wherein the extrusion die is maintained at a temperature of from about 420*F. to about 500*F. 